This invention generally relates to catheters, and particularly intravascular catheters for use in percutaneous transluminal coronary angioplasty (PTCA) or for the delivery of stents.
In a typical PTCA procedure, a dilatation balloon catheter is advanced over a guidewire to a desired location within the patient""s coronary anatomy where the balloon of the dilatation catheter is positioned within the stenosis to be dilated. The balloon is then inflated with radiopaque liquid at relatively high pressures (generally 4-16 atmospheres) to dilate the stenosed region of the diseased artery. One or more inflations may be needed to effectively dilate the stenosis.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion.
An essential step in effectively performing a PTCA procedure is properly positioning the balloon catheter at a desired location within the coronary artery. To properly position the balloon at the stenosed region, the catheter must have good pushability and flexibility, to be readily advanceable within the tortuous anatomy of the patient""s vasculature. Conventional catheter shafts may typically include multiple shaft sections having different flexibilities along the length of the catheter to improve the maneuverability of the catheter. A variety of methods have been used to form shafts having sections with variable flexibility, including forming multilayered shafts having transition sections in which a first layer tapers to a larger thickness as a second layer tapers inversely to a smaller thickness. One difficulty has been controlling the changes in the thickness of the first layer and second layer. Insufficient control, resulting in disadvantageously abrupt or gradual changes in the proportions of a first and second polymer, may result from conventional coextrusion systems, such as those using on/off valves to control the flow of the polymers making up the two layers. U.S. Pat. No. 5,725,814 (Harris), incorporated by reference herein in its entirety, discloses a coextrusion apparatus having gear pumps at the output ends of multiple extruders. The gear pumps are used to vary the relative amount of the polymers extruded and to thus form an extruded tube having multiple layers, the proportions of which vary along the length of the extruded tube.
What has been needed is a catheter which is highly trackable within the patient""s anatomy, with improved flexibility and pushability.
This invention is directed to a catheter having a shaft with at least two variable layers, wherein the relative proportions of the two layers change along at least a section of the multilayered shaft. The catheter shaft includes a section having a first layer tapering to a decreased thickness, and a second layer inversely tapering to an increased thickness, referred to herein as a cotapered transition section. The term inversely tapering should be understood to mean the second layer tapers in an opposite direction to the taper of the first layer. In one embodiment, the shaft has one or more tapered outer and/or inner surfaces in addition to the cotapered transition. The tapered outer or inner surface section of the shaft is typically longitudinally displaced from the cotapered transition section and may be directly adjacent to the cotapered transition section. In a presently preferred embodiment, the shaft having a cotapered transition section is coextruded. Another aspect of the invention includes a method of making a shaft having a cotapered transition section. The shaft of the invention provides improved transitions between shaft sections, to thereby change characteristics such as the shaft stiffness along the length of the catheter.
In one embodiment of the invention, the catheter shaft has a first longitudinal section having a cotapered transition section with a first layer having a tapered surface and a second layer having an inversely tapered surface, and a second longitudinal section with a tapered outer diameter. One of the first or the second layers has a tapered inner surface and a tapered outer surface in the second longitudinal section of the catheter shaft, and one of the first or second layers has a substantially constant wall thickness in the second longitudinal section of the catheter shaft. The first longitudinal section may be proximal or distal to the second longitudinal section, and the first layer may be an inner layer or an outer layer relative to the second layer.
In another embodiment of the invention, the catheter shaft has a first longitudinal section having a cotapered transition section with a first layer having a tapered surface and a second layer having an inversely tapered surface, and a second longitudinal section with a tapered outer diameter and with the first layer having a substantially constant wall thickness from at least a portion of the second longitudinal section to the distal end of the catheter shaft. In a presently preferred embodiment, the first longitudinal section is proximal to the second longitudinal section, and the first layer is an inner layer relative to the second layer.
In another embodiment, the catheter shaft has a first longitudinal section having a cotapered transition section with a first layer having a tapered surface and a second layer having an inversely tapered surface, and a second longitudinal section distal to and directly adjacent to the first longitudinal section, with a tapered outer diameter and with the first layer having an outer surface tapering distally to a smaller outer diameter and the second layer having an outer surface tapering distally to a smaller outer diameter. The first longitudinal section may be proximal or distal to the second longitudinal section, and the first layer may be an inner layer or an outer layer relative to the second layer. The first and second layers may be extend along the entire length of the shaft, or may have a proximal end and/or a distal end which terminate between the proximal and distal ends of the shaft.
In one embodiment, the catheter is a balloon catheter generally including an elongated shaft having a cotapered transition section in accordance with the invention and a balloon secured to a distal portion of the shaft.
A method of making an elongated multilayered catheter shaft having at least a first layer and a second layer and having a cotapered section in which the first layer tapers from a larger wall thickness to a smaller wall thickness and the second layer inversely tapers from a smaller wall thickness to a larger wall thickness, includes extruding the first layer from an extruder having a first pump at an output end thereof, and extruding the second layer from a second extruder having a second pump at an output end thereof. The first and second pumps are preferably gear pumps. The first and second layers are preferably coextruded through a coextrusion die downstream from the first and second extruders and first and second gear pumps. The direction of the first pump, e.g., the rotation of the first gear pump, is reversed to at least in part form the tapered surface in the first layer, and the speed of the second pump is increased from a first speed to a second speed to form the inverse taper in the second layer. As a result of temporarily setting the first gear pump into reverse rotation, plastic pressure is removed from the layer being tapered, and the time required to taper the first layer from the larger to the smaller wall thickness is advantageously reduced, to thereby produce a tailored, shorter transition. In one embodiment, forming the cotapered section includes temporarily speeding up the speed of the second gear pump from a first speed to a second speed which is greater than the steady state third speed used to form a section of the shaft after the cotapered section, to thereby reduce the time required to taper the second layer from the smaller wall thickness to the larger wall thickness. Thus, in one embodiment, a section of the shaft, which is adjacent and distal to the cotapered section, is formed by decreasing the speed of the second gear pump from the second speed used to form the cotapered section to a third speed greater than the first speed used to form a section of the shaft before the cotapered section and less than the second speed. The method of the invention provides improved short transitions varying the relative proportions of the first and second layers. The tailored short transitions are particularly advantageous in distal sections of the catheter shaft, to provide rapid changes in flexural modulus of the shaft for improved maneuverability of the catheter in the highly tortuous distal vascular anatomy.
Preferably, the polymer forming the first layer is different from the polymer forming the second layer. The polymer forming the inner layer of the cotapered section may have a hardness greater than, less than or equal to the hardness of the outer layer. In one embodiment, the viscosity of the polymers used to form the first and second layers is selected to minimize back pressure between the gear pumps and the die, and more specifically to minimize stored pressure energy which leads to long transitions.
The catheter shaft of the invention has excellent flexibility, pushability, and trackability due to the variable layers of the shaft having improved tapered transitions. The tapered transitions provide improved force transmission, cross, and flexibility in the distal end of the catheter, and tailoring of the catheter shaft characteristics, while avoiding the increased profile and increased risk of kinking in conventional fusion or adhesion junctions used to secure separate shaft sections together. The catheter shaft of the invention avoids significant negative effects on inflation/deflation time caused by disadvantageous decreases in the size of the shaft lumen(s) at bonded junctions. Additionally, one embodiment of the invention involves providing a catheter shaft having a tailored, short transition, providing improved rapid changes in the shaft flexural modulus while avoiding disadvantageously abrupt transitions. These and other advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying exemplary drawings.